What happens when you tighten a bolt? The bolt is pulled longer, “stretched”, and being springy it wants to return to its original length by pulling the joint together. The joint surfaces push back at the bolt keeping the bolt stretched. The bolt is strained in tension; the joint in compression; and our parts are firmly clamped together.
But how did we get the bolt to stretch? We tightened it, not only by stretching along its axis, but by twisting it. We stretched AND twisted the bolt. The stretch is necessary as it keeps our surfaces clamped together, but what is the twisting force doing? It might have gone into loosening our nut slightly the moment we released tension on our wrench; or it might still be locked into the joint in which case all of the wrench force you used to turn the nut may still be there trying to loosen the nut. It all depends on the friction between the nut face and the seating surface (an important lubrication consideration).
What happens if we use a torque wrench to “check the torque” on a bolt? If we torque the nut until we reach the “breakaway torque” we have to apply enough torque to overcome the friction between the nut face and joint surface. Until this happens none of our torque is felt by the bolt. Once the nut face is released, torque starts to twist the bolt until it overcomes the friction between the male and female threads. Only now does the nut move relative to the bolt and we detect the “breakaway” torque.
Notice that the only thing we are measuring is friction. Everything depends on friction. If the friction is more or less than when the bolt was originally tightened then our “breakaway” torque will be more or less than the torque that the assembler applied to the nut. That is the quandary. Did the assembler improperly torque the nut or did the friction change? We have no way of knowing.
“The nuts were loose.” Can we check assembly torque by loosening the nut? Same problem but only worse because if the twisting force is locked into our bolt then it might take only the smallest amount of torque to get the nut moving (our breakaway torque) as the bolt tries to unwind.
But what if the nut really is loose? Did the mechanic not tighten the nut properly? We still don't know. Take your pick; all or some of the reasons the nut might be loose from the list:
- embedment relaxation of the faying surfaces
- bolt stretch from metal creep (especially at high temperature)
- nut backed-off due to vibration loosening
- wasn’t tightened properly to begin with
- elastic interactions between multiple bolts in a flange has reduced preload (crosstalk).
Determining the degree of tightness in a joint by using a torque wrench to measure the breakaway torque is not accurate and leads to incorrect conclusions. Beware of inspectors carrying torque wrenches.